An index that tracks home-price changes in 20 U.S. metro areas dropped a record 16.3 percent in July compared to the same month last year.

The Standard & Poor’s/Case-Shiller Home Price 20-metro area index, which measures price changes for repeat sales of single-family homes, measured a 29.9 percent year-over-year price drop in the Las Vegas metro area — the most substantial drop on the list.